This invention relates to a process for the preparation of preserved transplants by dehydrating biological collagenous material.
In order to replace biological tissues, for instance skin, sinews, vessels, nerves, dura mater etc. presently autological tissues, in other words tissues arising from the same patient are being used. In order to produce them a second operation of the already weakened patient is in order, needing furthermore aseptical work and a large expenditure of time. An alternative to such a second operation may be found in the use of ready-made homologous or heterologous preserved tissue which may be used any time by the surgeon.
Freeze drying is a well known method for the preparation of biological transplants. In that case the water containing collagenous material is frozen at about -25.degree. C. and the water contained therein in the form of ice is removed by sublimation in vacuo. This method yields a collagenous tissue material containing a small percentage of water only, which may be kept in a sterile state as long as desired while keeping all its properties and may be used ready-for-use. The method of freeze-drying has several disadvantages when used for the preservation of planar materials.
Collagen is a structural protein, its fibrillar tissue swells when moist which causes an increase in thickness. The deep freezing so to say fixates the swollen state of the collagenous tissue. In case of a planar collagenous material, the dura mater for instance, a relatively thick spongy material results, the handling of which is restricted when used as a transplant. Its thickness, for instance amounts to 0.66 mm.
Ice crystals arising between the fibers and the fibrils loosen the collagenous fiber combination. The histological image shows clearly the difference between a freeze dried collagenous tissue and the respective tissue in its primary native condition. Due to the formation of ice crystals and the consecutive sublimation hollow spaces arise within the tissue, thereby changing its properties, elasticity for instance, compared to that of the native tissue. It worsens and is for native dura mater, for instance 1.8 kp, and for freeze dried dura mater, for instance 0.88 kp.
An additional criterion for the quality of a transplant particularly for prostethic plastics is the quality of the inner surface. The inner surface is graded according to its freeness of fibrils and fibers. The greater the freeness in other words the less partial adhesion or fusion of fiber to fiber occurs, the greater is the inner surface. The ideal exposure of the tissue is characterized by having a surface of about 20 m.sup.2 /g. The value for the inner surface is therefore an important criterion, because the transplanted tissue net functions better as a guiding means or grating for the ingrowing connective tissue, the more free fibers and fibrils occur in parallel regularity.
Due to the partial adhesion together of fibers and fibrils the inner surface of freeze dried Durakonserve (preserved dura mater) amounts to 5 to 10 m.sup.2 /g (measured by the nitrogen adsorption method).
Another method uses chemical preservation by Cialit. In that case the biological tissue is stored in an aqueous medium containing, for example 0.05% Cialit. A disadvantage of this method is that chemical substances adhere to the tissue which will be transferred to the locus of the operation.